1. Technical Field of the Invention
This invention relates to devices for manufacturing semiconductor wafers and, more particularly, to an automatic device for chemically and mechanically polishing the surface of semiconductor wafers.
2. Description of Related Art
Manufacturers of electronic semiconductor devices manufacture circular wafers of semiconductor material that generally have diameters ranging from four to eight inches (100-200 mm). The wafers are then cut into the sizes needed for various types of micro-processors or other semiconductor devices. The wafer substrate may be comprised of, for example, silicon, silica, simox, carbides, sapphire, or other compounds. Semiconductor device thin films may be applied to the surface of the substrate. Such thin films may be, for example, oxides, nitrides, metal oxides, polysilicon, EPI, ferroelectrics, or other metals.
The substrate/film combination must then be polished or finished to achieve a total surface flatness on the order of two microns or better. The polishing process must achieve this planar surface over the entire surface of the wafer so that semiconductor devices that are processed from any part of the wafer have the same relative flatness.
Existing systems for polishing the surface of semiconductor wafers are typified by the Model 372 Automatic Wafer Polisher from Westech Systems, Inc. The Model 372 performs the polishing process utilizing two polishing stations. A primary station performs material removal while a secondary station performs final polishing. The wafers are transported using an edge-contact shuttle, vacuum chuck, and water track.
FIG. 1 is a front side elevational view of an illustrative primary polishing station 1 according to the existing art. The primary station 1 is supplied with a polishing slurry 2 which is applied to a composite polishing pad 3 which covers the surface of a polishing pallet or table 4. The pallet 4 may have a diameter of approximately 30 inches, much larger than the semiconductor wafer. A template 5 holds the wafer 6 through a combination of side support and capillary action of water in the pores of a mounting material 7. The wafer is held in an inverted position, and is lowered from above until the wafer 6 contacts the polishing pad 3. The polishing action consists of three relative wafer motions. The template 5 holding the wafer 6 is rotated about its center, the template is oscillated across the diameter of the pallet 4, and the pallet 4 and pad 3 are rotated about the center of the pallet. This combination of motions is designed to remove material uniformly from the surface of the wafer 6. The final thickness of the wafer film is determined by estimating the amount of material removed over a given time period. More precise methods of measuring wafer thickness during the polishing process are needed.
Throughout the polishing process, critical variables include, among others, the type of polishing pad and its condition, the type of slurry, slurry PH, the condition of the slurry, the slurry flow, pad pressure, and the process temperature. Slight variations in any of these variables can, and often do, profoundly affect the outcome of the polishing process. For example, a worn or stretched polishing pad may cause major variations in the thickness of the wafer surface film. Even pads in good condition often rise up along the leading edge of the wafer as it passes over the pad. This also results in undesireable variations in wafer planarization from one area of the wafer to another. Termination of the polishing process is typically determined by polishing for a predetermined time period. However, this method results in inconsistencies in final film thickness from one wafer to the next due to changes in the variables outlined above. Better methods of controlling the removal of film material to achieve uniform wafer flatness, across the entire surface of a wafer and from one wafer to the next, are needed.
Although there are no known prior art teachings of a solution to the aforementioned deficiencies for polishing the surface of semiconductor wafers, there are a number of existing devices that are utilized in the computer hardware industry to abraid, burnish, and/or polish the surfaces of disks utilized in hard disk drives. Such devices are disclosed in U.S. Pat. No. 5,099,615 to Ruble et al., 5,065,547 to Shimizu et al., 4,736,475 to Ekhoff, and 4,347,689 to Hammond. Each of these references is discussed briefly below.
U.S. Pat. No. 5,099,615 to Ruble et al. discloses an automated rigid-disk finishing system for computer hard disks. The system includes an abrasive tape, a means for forcibly pressing the tape against the disk substrate, and a means for controlling the process to control the speed and tension of the tape. The disk has a hole in the center and is mounted on a spindle for rotation. As the disk is rotated, the abrasive tape is moved through the area of the tape/substrate interface thereby cutting concentric microscopic grooves into the substrate's surface. Both sides of the disk are simultaneous finished in this manner.
The Ruble device cannot, however, be utilized to polish the surface of semiconductor wafers for several reasons. First, only one side of semiconductor wafers is polished, and Ruble does not allow the mounting of a wafer for one-sided polishing. Second, semiconductor wafers do not have a hole in the center; therefore, the spindle mount utilized in Ruble will not function with semiconductor wafers. Finally, Ruble finishes the surface of the substrate with grooves cut in concentric circles. Semiconductor wafers, conversely, require an orbital polishing to polish the entire surface, including the area in the center of the wafer, to a uniform flatness rather than finishing the surface with concentric grooves.
U.S. Pat. No. 5,065,547 to Shimizu et al. discloses a surface processing machine utilizing a tape cartridge to polish or grind the surface of a computer hard disk. Shimizu, unlike Ruble, polishes only one side of a disk at a time. However, like Ruble, Shimizu utilizes a spindle mount for the hard disk and is therefore only suitable for disks which have a hole in the center. Therefore, Shimizu will not function with semiconductor wafers which do not have a central hole. Additionally, Shimizu polishes only in a concentric circular pattern, and is therefore, unsuitable for orbitally polishing semiconductor wafers.
U.S. Pat. No. 4,736,475 to Ekhoff discloses a surface finishing apparatus for disks which can hold more that one disk at a time, but otherwise has the same disadvantages and drawbacks as Ruble and Shimizu where semiconductor wafers are concerned.
U.S. Pat. No. 4,347,689 to Hammond discloses an apparatus for burnishing the coated recording surface of magnetic disks. Hammond polishes only one side of a disk at a time, utilizes a spindle through a central hole in the disk to hold the disk in place, and polishes in concentric circles. Therefore, for the reasons noted above, Hammond is also unsuitable for use with semiconductor wafers.
Review of each of the foregoing references reveals no disclosure or suggestion of a system or method such as that described and claimed herein.
It would be a distinct advantage to have an automated chemical and mechanical polishing (CMP) system for semiconductor wafers which polishes one side of a wafer, holds the wafer in place without requiring a central hole, polishes the surface of the wafer with an orbital motion, achieves a more accurate uniform thickness than existing CMP wafer polishing machines, and provides an accurate measurement of wafer thickness during the polishing process. The present invention provides such a system.